Liquid crystal display devices are being used widely as display device in clock, computer, will television and the like. The liquid display device is a display device using a liquid crystal, the molecular arrangement of which changes when only a slight voltage is applied thereto, thereby varying the direction of polarization, and the display device usually comprises two electrodes and a liquid crystal layer held between said electrodes.
In the liquid crystal display device of the type mentioned above, it is desirable that the distance between the electrodes, that is, a cell gap, is uniform.
If the cell gap formed in liquid crystal display device mentioned above is not uniform, the electric field applied to the liquid crystal layer comes to have partial irregularity in strength. On that account, the contrast ratio of the image formed on the display device varies depending on the spot, whereby irregular density are formed in the image, or alternatively irregular color of the image happens when said image is formed by such a display system as STN. Further, the response speed of liquid crystal to an input signal varies regularly according to the cell gap and strength of the electric field, but if the cell gap is not uniform, said response speed varies irregularly, failing to give clear images.
For that reason, it was a common practice in the field of liquid crystal display devices to form a uniform liquid crystal layer between a pair of electrodes in a liquid crystal device by charging a liquid crystal into a space between a pair of electrodes wherein a particulate spacer composed of an insulator is lying.
For example, Japanese Patent L-0-P Publn. No. 78227/1989 proposes that the use as the above-mentioned spacer of spherical particles having a standard deviation of not more than 0.3 .mu.m dispersed between a pair of electrodes in order to prevent discoloration from displayed image.
Further, Japanese Patent L-0-P Publn. No. 269933/1987 discloses the use as a spacer for a liquid crystal display panel of silica particles having a coefficient of variation (CV value) of not more than 10.0%, said CV value indicating uniformity of particle diameter of said silica particles, and in the example of said patent publication, there are used silica particles having a CV value of 8%.
When spacer particles having such standard deviation and CV value as defined in the cited publications mentioned above are used in liquid crystal display devices of the ordinary type, it is possible to prevent displayed images from irregular density or irregular color to a certain extent without relying upon the length of the cell gap. However, in the case of a liquid crystal display device using such a liquid crystal as a ferroelectric liquid crystal and having a small cell gap such as 1 to 2 .mu.m, the cell gap to be adopted therein is required to be smaller in gap irregularity. Further, in the case of a liquid crystal display device using STN liquid crystal, if the cell gap is large in length, a specific interference color is unobtainable, resulting in displaying irregularities, hence the cell gap adopted therein is required to be uniform all the more.
Extensive researches conducted by the present inventors with the view of obtaining liquid crystal display devices free from displaying irregularities have resulted in the finding that such excellent liquid crystal display devices as aimed at above are unobtainable by the use of spherical particles merely having a small CV value mentioned above as the spacer for liquid crystal display cell.
That is, it has been found that even when the spherical particles used have a CV value of not more than 10.0%, if the proportion of aggregated particles becomes large, irregularities of the cell gap take place and, at the same time, cold bubbles are formed.
By cold bubbles as used herein are meant air bubble-like matters formed in the cell gap in the following manner.
That is, when a liquid display device is exposed to such a low temperature of below an ice point (0.degree. C.), by shrinkage of the liquid crystal sandwiched between a pair of transparent electrodes, space portions are formed partially between at least one of the transparent electrodes and liquid crystal and these space portions remain as so-called "cold bubbles" even when the liquid crystal display device is warmed to room temperature.
The present invention is intended to solve such problems associated with the prior art as mentioned above, and an object of the invention is to provide liquid crystal display devices in which irregular density and irregular color of the displayed images are inhibited assuredly all the more and, moreover, no cold bubbles occur.